Multi-color electrophoretic displays

ABSTRACT

A multi-color electrophoretic medium contains first, second and third species of particles. The first species of particles is light-scattering, while the second and third species of particles are transmissive. A method for driving such a display is also described.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 14/987,191,filed Jan. 4, 2016, which is a continuation of application Ser. No.13/892,517, filed May 13, 2013, now U.S. Pat. No. 9,268,191, issued Feb.23, 2016, which is a divisional of application Ser. No. 13/251,457,filed Oct. 3, 2011 now U.S. Pat. No. 8,441,714, issued May 14, 2013,which is continuation of application Ser. No. 12/725,997, filed Mar. 17,2010, now U.S. Pat. No. 8,040,594, issued Oct. 18, 2011, which is acontinuation-in-part of application Ser. No. 12/117,974, filed May 9,2008, now U.S. Pat. No. 7,791,789, issued Sep. 7, 2010, which is acontinuation of application Ser. No. 11/970,811, filed Jan. 8, 2008, nowU.S. Pat. No. 8,384,658, issued Feb. 26, 2013, which is a continuationof application Ser. No. 10/729,044, filed Dec. 5, 2003, now U.S. Pat.No. 7,352,353, issued Apr. 1, 2008, the contents of all of which areincorporated herein by reference.

BACKGROUND OF INVENTION

The present invention relates to multi-color electrophoretic media andto displays incorporating such media.

Traditionally, electronic displays such as liquid crystal displays havebeen made by sandwiching an optoelectrically active material between twopieces of glass. In many cases each piece of glass has an etched, clearelectrode structure formed using indium tin oxide. A first electrodestructure controls all the segments of the display that may beaddressed, that is, changed from one visual state to another. A secondelectrode, sometimes called a counter electrode, addresses all displaysegments as one large electrode, and is generally designed not tooverlap any of the rear electrode wire connections that are not desiredin the final image. Alternatively, the second electrode is alsopatterned to control specific segments of the displays. In thesedisplays, unaddressed areas of the display have a defined appearance.

Electrophoretic display media, generally characterized by the movementof particles through an applied electric field, are highly reflective,can be made bistable, and consume very little power. Such displays canhave attributes of good brightness and contrast, wide viewing angles,state bistability, and low power consumption when compared with liquidcrystal displays. Encapsulated electrophoretic displays also enable thedisplay to be printed. These properties allow encapsulatedelectrophoretic display media to be used in many applications for whichtraditional electronic displays are not suitable, such as flexibledisplays. The electro-optical properties of encapsulated displays allow,and in some cases require, novel schemes or configurations to be used toaddress the displays. Accordingly, such displays have been the subjectof intense research and development for a number of years.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

Nevertheless, problems with the long-term image quality ofnon-encapsulated electrophoretic displays have prevented theirwidespread usage. For example, particles that make up electrophoreticdisplays tend to settle, resulting in inadequate service-life for thesedisplays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporationdescribe various technologies used in encapsulated electrophoretic andother electro-optic media. Such encapsulated media comprise numeroussmall capsules, each of which itself comprises an internal phasecontaining electrophoretically-mobile particles in a fluid medium, and acapsule wall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thethese patents and applications include:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. No. 7,002,728; and U.S. Patent Application        Publication No. 2007/0146310;    -   (b) Capsules, binders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276 and; 7,411,719;    -   (c) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. No. 6,982,178; and U.S. Patent        Application Publication No. 2007/0109219;    -   (d) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. No.        7,116,318; and U.S. Patent Application Publication No.        2007/0035808;    -   (e) Color formation and color adjustment; see for example U.S.        Pat. Nos. 6,017,584; 6,664,944; 6,864,875; 7,075,502; and        7,167,155; and U.S. Patent Applications Publication Nos.        2004/0190114; 2004/0263947; 2007/0109219; 2007/0223079;        2008/0023332; 2008/0043318; and 2008/0048970;    -   (f) Methods for driving displays; see for example U.S. Pat. No.        7,012,600; and U.S. Patent Application Publication No.        2006/0262060;    -   (g) Applications of displays; see for example U.S. Pat. No.        7,312,784; and U.S. Patent Application Publication No.        2006/0279527; and    -   (h) Non-electrophoretic displays, as described in U.S. Pat. Nos.        6,241,921; 6,950,220; and 7,420,549.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,U.S. Pat. No. 6,866,760. Accordingly, for purposes of the presentapplication, such polymer-dispersed electrophoretic media are regardedas sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Electrophoretic media operating in shutter mode may be useful inmulti-layer structures for full color displays; in such structures, atleast one layer adjacent the viewing surface of the display operates inshutter mode to expose or conceal a second layer more distant from theviewing surface.

As already indicated, an encapsulated or microcell electrophoreticdisplay typically does not suffer from the clustering and settlingfailure mode of traditional electrophoretic devices and provides furtheradvantages, such as the ability to print or coat the display on a widevariety of flexible and rigid substrates. (Use of the word “printing” isintended to include all forms of printing and coating, including, butwithout limitation: pre-metered coatings such as patch die coating, slotor extrusion coating, slide or cascade coating, curtain coating; rollcoating such as knife over roll coating, forward and reverse rollcoating; gravure coating; dip coating; spray coating; meniscus coating;spin coating; brush coating; air knife coating; silk screen printingprocesses; electrostatic printing processes; thermal printing processes;ink jet printing processes; electrophoretic deposition (See U.S. Pat.No. 7,339,715); and other similar techniques.) Thus, the resultingdisplay can be flexible. Further, because the display medium can beprinted (using a variety of methods), the display itself can be madeinexpensively.

Most prior art electrophoretic media essentially display only twocolors. Such electrophoretic media either use a single type ofelectrophoretic particle having a first color in a colored fluid havinga second, different color (in which case, the first color is displayedwhen the particles lie adjacent the viewing surface of the display andthe second color is displayed when the particles are spaced from theviewing surface), or first and second types of electrophoretic particleshaving differing first and second colors in an uncolored fluid (in whichcase, the first color is displayed when the first type of particles lieadjacent the viewing surface of the display and the second color isdisplayed when the second type of particles lie adjacent the viewingsurface). Typically the two colors are black and white. If a full colordisplay is desired, a color filter array may be disposed over theviewing surface of the monochrome (black and white) display. Such acolor filter array is typically of the red/green/blue (“RGB”) orred/green/blue/white (“RGBW”) type. Displays with color filters relyupon an area sharing approach with three (in the case of RGB displays)or four (in the case of RGBW displays) sub-pixels together functioningas a single full color pixel. Unfortunately, each color can only bedisplayed by part of the display area. For example, in an RGBW display,each of red, green and blue can only be displayed by ¼ of the displayarea (one sub-pixel out of four) and white can effectively be displayedby ½ of the display area (one complete sub-pixel out of four, plus eachcolored sub-pixel acts as ⅓ white, so the three colored sub-pixelstogether provide another one complete white sub-pixel). This areasharing approach result in colors less bright than is desirable.

Alternatively full color displays can be constructed using multiplecolor-changing layers, with at least one front (i.e., adjacent theviewing surface) color-changing layer operating in shutter mode. Apartfrom being complicated and potentially expensive, such a multi-layerdisplay requires precise alignment of the various layers, and highlylight transmissive electrodes (and transistors, in the case of an activematrix display).

The aforementioned U.S. Pat. No. 6,017,584 describes an electrophoreticmedium having three different types of particles having three differentcolors in a colored or uncolored fluid, and a method of driving theparticles so as to enable each of the three different colors to bedisplayed. The relevant disclosure from the aforementioned U.S. Pat. No.6,017,584 is reproduced below with reference to FIGS. 6-9 of theaccompanying drawings.

There is still, however, a need for electrophoretic media capable ofdisplaying more colors at each pixel in order that, for example, suchmedia can reproduce the appearance of high quality color printing. Suchhigh quality printing is typically effected using at least four inks,cyan/magenta/yellow/black (“CMYK”). It is often not appreciated that aso-called “four-color” CMYK printing system is in reality a five-colorsystem, the fifth color being the white background provided by the paper(or similar) surface when no ink is applied thereto. Since there is nocomparable background color in an essentially opaque electrophoreticmedium unless it is being used in shutter mode, a non-shutter modeelectrophoretic medium should be capable of displaying five colors(black, white and three primary colors, the three primary colorstypically being cyan, magenta and yellow. It has now been realized thatby this aim can be achieved by using the electrophoretic medium from theaforementioned U.S. Pat. No. 6,017,584 having three different types ofparticles in a colored fluid and choosing the colors of both theparticles and the fluid carefully.

SUMMARY OF INVENTION

An object of the invention is to provide a highly-flexible, reflectivedisplay which can be manufactured easily, consumes little (or no in thecase of bistable displays) power, and can, therefore, be incorporatedinto a variety of applications. The invention features a printabledisplay comprising an encapsulated electrophoretic display medium. Theresulting display is flexible. Since the display media can be printed,the display itself can be made inexpensively.

An encapsulated electrophoretic display can be constructed so that theoptical state of the display is stable for some length of time. When thedisplay has two states which are stable in this manner, the display issaid to be bistable. If more than two states of the display are stable,then the display can be said to be multistable. For the purpose of thisinvention, the term bistable will be used to indicate a display in whichany optical state remains fixed once the addressing voltage is removed.The definition of a bistable state depends on the application for thedisplay. A slowly-decaying optical state can be effectively bistable ifthe optical state is substantially unchanged over the required viewingtime. For example, in a display which is updated every few minutes, adisplay image which is stable for hours or days is effectively bistablefor that application. In this invention, the term bistable alsoindicates a display with an optical state sufficiently long-lived as tobe effectively bistable for the application in mind. Alternatively, itis possible to construct encapsulated electrophoretic displays in whichthe image decays quickly once the addressing voltage to the display isremoved (i.e., the display is not bistable or multistable). As will bedescribed, in some applications it is advantageous to use anencapsulated electrophoretic display which is not bistable. Whether ornot an encapsulated electrophoretic display is bistable, and its degreeof bistability, can be controlled through appropriate chemicalmodification of the electrophoretic particles, the suspending fluid, thecapsule, and binder materials.

An encapsulated electrophoretic display may take many forms. The displaymay comprise capsules dispersed in a binder. The capsules may be of anysize or shape. The capsules may, for example, be spherical and may havediameters in the millimeter range or the micron range, but is preferablyfrom ten to a few hundred microns. The capsules may be formed by anencapsulation technique, as described below. Particles may beencapsulated in the capsules. The particles may be two or more differenttypes of particles. The particles may be colored, luminescent,light-absorbing or transparent, for example. The particles may includeneat pigments, dyed (laked) pigments or pigment/polymer composites, forexample. The display may further comprise a suspending fluid in whichthe particles are dispersed.

The successful construction of an encapsulated electrophoretic displayrequires the proper interaction of several different types of materialsand processes, such as a polymeric binder and, optionally, a capsulemembrane. These materials must be chemically compatible with theelectrophoretic particles and fluid, as well as with each other. Thecapsule materials may engage in useful surface interactions with theelectrophoretic particles, or may act as a chemical or physical boundarybetween the fluid and the binder.

In some cases, the encapsulation step of the process is not necessary,and the electrophoretic fluid may be directly dispersed or emulsifiedinto the binder (or a precursor to the binder materials) and aneffective “polymer-dispersed electrophoretic display” constructed. Insuch displays, voids created in the binder may be referred to ascapsules or microcapsules even though no capsule membrane is present.The binder dispersed electrophoretic display may be of the emulsion orphase separation type.

Throughout the specification, reference will be made to printing orprinted. As used throughout the specification, printing is intended toinclude all forms of printing and coating, including: premeteredcoatings such as patch die coating, slot or extrusion coating, slide orcascade coating, and curtain coating; roll coating such as knife overroll coating, forward and reverse roll coating; gravure coating; dipcoating; spray coating; meniscus coating; spin coating; brush coating;air knife coating; silk screen printing processes; electrostaticprinting processes; thermal printing processes; and other similartechniques. A “printed element” refers to an element formed using anyone of the above techniques.

This invention provides novel methods and apparatus for controlling andaddressing particle-based displays. Additionally, the inventiondiscloses applications of these methods and materials on flexiblesubstrates, which are useful in large-area, low cost, or high-durabilityapplications.

In one aspect, the present invention relates to an encapsulatedelectrophoretic display. The display includes a substrate and at leastone capsule containing a highly-resistive fluid and a plurality ofparticles. The display also includes at least two electrodes disposedadjacent the capsule, a potential difference between the electrodescausing some of the particles to migrate toward at least one of the twoelectrodes. This causes the capsule to change optical properties.

In another aspect, the present invention relates to a coloredelectrophoretic display. The electrophoretic display includes asubstrate and at least one capsule containing a highly-resistive fluidand a plurality of particles. The display also includes coloredelectrodes. Potential differences are applied to the electrodes in orderto control the particles and present a colored display to a viewer.

In yet another aspect, the present invention relates to anelectrostatically addressable display comprising a substrate, anencapsulated electrophoretic display adjacent the substrate, and anoptional dielectric sheet adjacent the electrophoretic display.Application of an electrostatic charge to the dielectric sheet ordisplay material modulates the appearance of the electrophoreticdisplay.

In still another aspect, the present invention relates to anelectrostatically addressable encapsulated display comprising a film anda pair of electrodes. The film includes at least one capsule containingan electrophoretic suspension. The pair of electrodes is attached toeither side of the film. Application of an electrostatic charge to thefilm modulates the optical properties.

In still another aspect, the present invention relates to anelectrophoretic display that comprises a conductive substrate, and atleast one capsule printed on such substrate. Application of anelectrostatic charge to the capsule modulates the optical properties ofthe display.

In still another aspect the present invention relates to a method formatrix addressing an encapsulated display. The method includes the stepof providing three or more electrodes for each display cell and applyinga sequence of potentials to the electrodes to control movement ofparticles within each cell.

In yet another aspect, the present invention relates to a matrixaddressed electrophoretic display. The display includes a capsulecontaining charged particles and three or more electrodes disposedadjacent the capsule. A sequence of voltage potentials is applied to thethree or more electrodes causing the charged particles to migrate withinthe capsule responsive to the sequence of voltage potentials.

In still another aspect, the present invention relates to a rearelectrode structure for electrically addressable displays. The structureincludes a substrate, a first electrode disposed on a first side of thesubstrate, and a conductor disposed on a second side of the substrate.The substrate defines at least one conductive via in electricalcommunication with both the first electrode and the conductor.

In yet another aspect, this invention provides a multi-colorelectrophoretic medium containing at least first, second and thirdspecies of particles, the particles having substantially non-overlappingelectrophoretic mobilities and first, second and third colorsrespectively, the first, second and third colors differing from eachother, the particles being dispersed in a fluid having a fourth colordifferent from the first, second and third colors.

In such a multi-color medium, the first, second, third and fourth colorsmay be cyan, magenta, yellow and white, in any order. Since it is verydifficult to make a satisfactory white (light-scattering) fluid, it isgenerally preferred that one of the three types of particles have awhite color. As already noted, the first, second and third types ofparticles must having differing (and non-zero) electrophoreticmobilities. Although in principle all three types of particles couldbear charges of the same polarity but differing magnitudes to providethe differing electrophoretic mobilities, it is generally moreconvenient to have two types of particles bearing charges of onepolarity, with the other type of particles bear charges of the oppositepolarity. When, as in one preferred form of the invention, one of thethree types of particles is white, it is preferred that the whiteparticle bear charges of one polarity and that the other two types ofparticles (conveniently cyan and magenta) bear charges of the oppositepolarity.

As already mentioned, many preferred embodiments of the invention willhave three types of particles colored white and two other colors. Bothtransmissive and reflective colored particles can be used in the presentinvention. White particles operate by scattering light and hence areessentially reflective; a “transmissive” white particle would beessentially transparent and hence not useful in the present invention.However, as illustrated in the drawings and described below, bothtransmissive and reflective particles having colors other than white canbe used, although the positioning of the various particles, andespecially the positioning of the white particles, needed to producevarious colors varies depending upon whether transmissive or reflectivenon-white particles are used.

The electrophoretic medium of the present invention may be of theencapsulated type and comprise a capsule wall within which the fluid andthe electrically charged particles are retained. Such an encapsulatedmedium may comprise a plurality of capsules each comprising a capsulewall, with the fluid and electrically charged particle retained therein,the medium further comprising a polymeric binder surrounding thecapsules. Alternatively, the medium may be of the microcell orpolymer-dispersed types discussed above.

This invention extends to an electrophoretic display comprising anelectrophoretic medium of the present invention and at least oneelectrode disposed adjacent the electrophoretic medium for applying anelectric field to the medium. The displays of the present invention maybe used in any application in which prior art electro-optic displayshave been used. Thus, for example, the present displays may be used inelectronic book readers, portable computers, tablet computers, cellulartelephones, smart cards, signs, watches, shelf labels and flash drives.

In yet another aspect, this invention provides a method of driving amulti-color electrophoretic display containing at least first, secondand third species of particles, the particles having substantiallynon-overlapping electrophoretic mobilities and first, second and thirdcolors respectively, the first, second and third colors differing fromeach other, the particles being dispersed in a fluid having a fourthcolor different from the first, second and third colors, the displayfurther comprising a first electrode forming a viewing surface of thedisplay and a second electrode on the opposed side of the fluid from thefirst electrode, the method comprising:

-   -   bringing all three species of particles adjacent one of the        first and second electrodes;    -   applying an electric field between the first and second        electrodes to cause at least one species of particles to move        away from said one electrode, thereby placing a desired one of        the three species of particles adjacent the viewing surface; and    -   applying an electric field between the first and second        electrodes to cause all three species of particles to move away        from the first electrode, whereby the fourth color of the fluid        is displayed at the viewing surface.

BRIEF DESCRIPTION OF DRAWINGS

The invention is pointed out with particularity in the appended claims.The advantages of the invention described above, together with furtheradvantages, may be better understood by referring to the followingdescription taken in conjunction with the accompanying drawings. In thedrawings, like reference characters generally refer to the same partsthroughout the different views. Also, the drawings are not necessarilyto scale, emphasis instead generally being placed upon illustrating theprinciples of the invention.

FIG. 1A is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich the smaller electrode has been placed at a voltage relative to thelarge electrode causing the particles to migrate to the smallerelectrode.

FIG. 1B is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich the larger electrode has been placed at a voltage relative to thesmaller electrode causing the particles to migrate to the largerelectrode.

FIG. 1C is a diagrammatic top-down view of one embodiment of arear-addressing electrode structure.

FIG. 2A is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerassociated with the larger electrode in which the smaller electrode hasbeen placed at a voltage relative to the large electrode causing theparticles to migrate to the smaller electrode.

FIG. 2B is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerassociated with the larger electrode in which the larger electrode hasbeen placed at a voltage relative to the smaller electrode causing theparticles to migrate to the larger electrode.

FIG. 2C is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerdisposed below the larger electrode in which the smaller electrode hasbeen placed at a voltage relative to the large electrode causing theparticles to migrate to the smaller electrode.

FIG. 2D is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerdisposed below the larger electrode in which the larger electrode hasbeen placed at a voltage relative to the smaller electrode causing theparticles to migrate to the larger electrode.

FIG. 3A is a diagrammatic side view of an embodiment of an addressingstructure in which a direct-current electric field has been applied tothe capsule causing the particles to migrate to the smaller electrode.

FIG. 3B is a diagrammatic side view of an embodiment of an addressingstructure in which an alternating-current electric field has beenapplied to the capsule causing the particles to disperse into thecapsule.

FIG. 3C is a diagrammatic side view of an embodiment of an addressingstructure having transparent electrodes, in which a direct-currentelectric field has been applied to the capsule causing the particles tomigrate to the smaller electrode.

FIG. 3D is a diagrammatic side view of an embodiment of an addressingstructure having transparent electrodes, in which an alternating-currentelectric field has been applied to the capsule causing the particles todisperse into the capsule.

FIG. 4A is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich multiple smaller electrodes have been placed at a voltage relativeto multiple larger electrodes, causing the particles to migrate to thesmaller electrodes.

FIG. 4B is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich multiple larger electrodes have been placed at a voltage relativeto multiple smaller electrodes, causing the particles to migrate to thelarger electrodes.

FIG. 5A is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display havingcolored electrodes and a white electrode, in which the coloredelectrodes have been placed at a voltage relative to the white electrodecausing the particles to migrate to the colored electrodes.

FIG. 5B is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display havingcolored electrodes and a white electrode, in which the white electrodehas been placed at a voltage relative to the colored electrodes causingthe particles to migrate to the white electrode.

FIG. 6 is a diagrammatic side view of an embodiment of a color displayelement having red, green, and blue particles of differentelectrophoretic mobilities.

FIGS. 7A-7B depict the steps taken to address the display of FIG. 6 todisplay red.

FIGS. 8A-8D depict the steps taken to address the display of FIG. 6 todisplay blue.

FIGS. 9A-9C depict the steps taken to address the display of FIG. 6 todisplay green.

FIGS. 10A-10H depict a color display element having white, cyan andmagenta particles of different electrophoretic mobilities in a yellowcolored fluid, the cyan and magenta particles being reflective, andillustrate respectively, the white, cyan, magenta, yellow, red, green,blue and black optical states of the display.

FIGS. 11A-11H depict a color display element similar to that shown inFIGS. 10A-10H but in which the cyan and magenta particles aretransmissive, with FIGS. 11A-11H illustrating the same optical states asFIGS. 10A-10H respectively.

FIG. 12 is a perspective embodiment of a rear electrode structure foraddressing a seven segment display.

FIG. 13 is a perspective embodiment of a rear electrode structure foraddressing a three by three matrix display element.

FIG. 14 is a cross-sectional view of a printed circuit board used as arear electrode addressing structure.

FIG. 15 is a cross-sectional view of a dielectric sheet used as a rearelectrode addressing structure.

FIG. 16 is a cross-sectional view of a rear electrode addressingstructure that is formed by printing.

FIG. 17 is a perspective view of an embodiment of a control gridaddressing structure.

FIG. 18 is an embodiment of an electrophoretic display that can beaddressed using a stylus.

DETAILED DESCRIPTION

An electronic ink is an optoelectronically active material whichcomprises at least two phases: an electrophoretic contrast media phaseand a coating/binding phase. The electrophoretic phase comprises, insome embodiments, a single species of electrophoretic particlesdispersed in a clear or dyed medium, or more than one species ofelectrophoretic particles having distinct physical and electricalcharacteristics dispersed in a clear or dyed medium. In some embodimentsthe electrophoretic phase is encapsulated, that is, there is a capsulewall phase between the two phases. The coating/binding phase includes,in one embodiment, a polymer matrix that surrounds the electrophoreticphase. In this embodiment, the polymer in the polymeric binder iscapable of being dried, crosslinked, or otherwise cured as intraditional inks, and therefore a printing process can be used todeposit the electronic ink onto a substrate. An electronic ink iscapable of being printed by several different processes, depending onthe mechanical properties of the specific ink employed. For example, thefragility or viscosity of a particular ink may result in a differentprocess selection. A very viscous ink would not be well-suited todeposition by an inkjet printing process, while a fragile ink might notbe used in a knife over roll coating process.

The optical quality of an electronic ink is quite distinct from otherelectronic display materials. The most notable difference is that theelectronic ink provides a high degree of both reflectance and contrastbecause it is pigment based (as are ordinary printing inks). The lightscattered from the electronic ink comes from a very thin layer ofpigment close to the top of the viewing surface. In this respect itresembles an ordinary, printed image. Also, electronic ink is easilyviewed from a wide range of viewing angles in the same manner as aprinted page, and such ink approximates a Lambertian contrast curve moreclosely than any other electronic display material. Since electronic inkcan be printed, it can be included on the same surface with any otherprinted material, including traditional inks. Electronic ink can be madeoptically stable in all display configurations, that is, the ink can beset to a persistent optical state. Fabrication of a display by printingan electronic ink is particularly useful in low power applicationsbecause of this stability.

Electronic ink displays are novel in that they can be addressed by DCvoltages and draw very little current. As such, the conductive leads andelectrodes used to deliver the voltage to electronic ink displays can beof relatively high resistivity. The ability to use resistive conductorssubstantially widens the number and type of materials that can be usedas conductors in electronic ink displays. In particular, the use ofcostly vacuum-sputtered indium tin oxide (ITO) conductors, a standardmaterial in liquid crystal devices, is not required. Aside from costsavings, the replacement of ITO with other materials can providebenefits in appearance, processing capabilities (printed conductors),flexibility, and durability. Additionally, the printed electrodes are incontact only with a solid binder, not with a fluid layer (like liquidcrystals). This means that some conductive materials, which wouldotherwise dissolve or be degraded by contact with liquid crystals, canbe used in an electronic ink application. These include opaque metallicinks for the rear electrode (e.g., silver and graphite inks), as well asconductive transparent inks for either substrate. These conductivecoatings include semiconducting colloids, examples of which are indiumtin oxide and antimony-doped tin oxide. Organic conductors (polymericconductors and molecular organic conductors) also may be used. Polymersinclude, but are not limited to, polyaniline and derivatives,polythiophene and derivatives, poly(3,4-ethylenedioxythiophene) (PEDOT)and derivatives, polypyrrole and derivatives, and polyphenylenevinylene(PPV) and derivatives. Organic molecular conductors include, but are notlimited to, derivatives of naphthalene, phthalocyanine, and pentacene.Polymer layers can be made thinner and more transparent than withtraditional displays because conductivity requirements are not asstringent.

As an example, there are classes of materials called electroconductivepowders which are also useful as coatable transparent conductors inelectronic ink displays. One example is Zelec ECP electroconductivepowders from DuPont Chemical Co. of Wilmington, Del.

Referring now to FIGS. 1A and 1B, an addressing scheme for controllingparticle-based displays is shown in which electrodes are disposed ononly one side of a display, allowing the display to be rear-addressed.Utilizing only one side of the display for electrodes simplifiesfabrication of displays. For example, if the electrodes are disposed ononly the rear side of a display, both of the electrodes can befabricated using opaque materials, because the electrodes do not need tobe transparent.

FIG. 1A depicts a single capsule 20 of an encapsulated display media. Inbrief overview, the embodiment depicted in FIG. 1A includes a capsule 20containing at least one particle 50 dispersed in a suspending fluid 25.The capsule 20 is addressed by a first electrode 30 and a secondelectrode 40. The first electrode 30 is smaller than the secondelectrode 40. The first electrode 30 and the second electrode 40 may beset to voltage potentials which affect the position of the particles 50in the capsule 20.

The particles 50 represent 0.1% to 20% of the volume enclosed by thecapsule 20. In some embodiments the particles 50 represent 2.5% to 17.5%of the volume enclosed by capsule 20. In preferred embodiments, theparticles 50 represent 5% to 15% of the volume enclosed by the capsule20. In more preferred embodiments the particles 50 represent 9% to 11%of the volume defined by the capsule 20. In general, the volumepercentage of the capsule 20 that the particles 50 represent should beselected so that the particles 50 expose most of the second, largerelectrode 40 when positioned over the first, smaller electrode 30. Asdescribed in detail below, the particles 50 may be colored any one of anumber of colors. The particles 50 may be either positively charged ornegatively charged.

The particles 50 are dispersed in a dispersing fluid 25. The dispersingfluid 25 should have a low dielectric constant. The fluid 25 may beclear, or substantially clear, so that the fluid 25 does not inhibitviewing the particles 50 and the electrodes 30, 40 from position 10. Inother embodiments, the fluid 25 is dyed. In some embodiments thedispersing fluid 25 has a specific gravity matched to the density of theparticles 50. These embodiments can provide a bistable display media,because the particles 50 do not tend to move in certain compositionsabsent an electric field applied via the electrodes 30, 40.

The electrodes 30, 40 should be sized and positioned appropriately sothat together they address the entire capsule 20. There may be exactlyone pair of electrodes 30, 40 per capsule 20, multiple pairs ofelectrodes 30, 40 per capsule 20, or a single pair of electrodes 30, 40may span multiple capsules 20. In the embodiment shown in FIGS. 1A and1B, the capsule 20 has a flattened, rectangular shape. In theseembodiments, the electrodes 30, 40 should address most, or all, of theflattened surface area adjacent the electrodes 30, 40. The smallerelectrode 30 is at most one-half the size of the larger electrode 40. Inpreferred embodiments the smaller electrode is one-quarter the size ofthe larger electrode 40; in more preferred embodiments the smallerelectrode 30 is one-eighth the size of the larger electrode 40. In evenmore preferred embodiments, the smaller electrode 30 is one-sixteenththe size of the larger electrode 40. It should be noted that referenceto “smaller” in connection with the electrode 30 means that theelectrode 30 addresses a smaller amount of the surface area of thecapsule 20, not necessarily that the electrode 30 is physically smallerthan the larger electrode 40. For example, multiple capsules 20 may bepositioned such that less of each capsule 20 is addressed by the“smaller” electrode 30, even though both electrodes 30, 40 are equal insize. It should also be noted that, as shown in FIG. 1C, electrode 30may address only a small corner of a rectangular capsule 20 (shown inphantom view in FIG. 1C), requiring the larger electrode 40 to surroundthe smaller electrode 30 on two sides in order to properly address thecapsule 20. Selection of the percentage volume of the particles 50 andthe electrodes 30, 40 in this manner allow the encapsulated displaymedia to be addressed as described below.

Electrodes may be fabricated from any material capable of conductingelectricity so that electrode 30, 40 may apply an electric field to thecapsule 20. As noted above, the rear-addressed embodiments depicted inFIGS. 1A and 1B allow the electrodes 30, 40 to be fabricated from opaquematerials such as solder paste, copper, copper-clad polyimide, graphiteinks, silver inks and other metal-containing conductive inks.Alternatively, electrodes may be fabricated using transparent materialssuch as indium tin oxide and conductive polymers such as polyaniline orpolythiophenes. Electrodes 30, 40 may be provided with contrastingoptical properties. In some embodiments, one of the electrodes has anoptical property complementary to optical properties of the particles50.

In one embodiment, the capsule 20 contains positively charged blackparticles 50, and a substantially clear suspending fluid 25. The first,smaller electrode 30 is colored black, and is smaller than the secondelectrode 40, which is colored white or is highly reflective. When thesmaller, black electrode 30 is placed at a negative voltage potentialrelative to larger, white electrode 40, the positively-charged particles50 migrate to the smaller, black electrode 30. The effect to a viewer ofthe capsule 20 located at position 10 is a mixture of the larger, whiteelectrode 40 and the smaller, black electrode 30, creating an effectwhich is largely white. Referring to FIG. 1B, when the smaller, blackelectrode 30 is placed at a positive voltage potential relative to thelarger, white electrode 40, particles 50 migrate to the larger, whiteelectrode 40 and the viewer is presented a mixture of the blackparticles 50 covering the larger, white electrode 40 and the smaller,black electrode 30, creating an effect which is largely black. In thismanner the capsule 20 may be addressed to display either a white visualstate or a black visual state.

Other two-color schemes are easily provided by varying the color of thesmaller electrode 30 and the particles 50 or by varying the color of thelarger electrode 40. For example, varying the color of the largerelectrode 40 allows fabrication of a rear-addressed, two-color displayhaving black as one of the colors. Alternatively, varying the color ofthe smaller electrode 30 and the particles 50 allow a rear-addressedtwo-color system to be fabricated having white as one of the colors.Further, it is contemplated that the particles 50 and the smallerelectrode 30 can be different colors. In these embodiments, a two-colordisplay may be fabricated having a second color that is different fromthe color of the smaller electrode 30 and the particles 50. For example,a rear-addressed, orange-white display may be fabricated by providingblue particles 50, a red, smaller electrode 30, and a white (or highlyreflective) larger electrode 40. In general, the optical properties ofthe electrodes 30, 40 and the particles 50 can be independently selectedto provide desired display characteristics. In some embodiments theoptical properties of the dispersing fluid 25 may also be varied, e.g.the fluid 25 may be dyed.

In other embodiments the larger electrode 40 may be reflective insteadof white. In these embodiments, when the particles 50 are moved to thesmaller electrode 30, light reflects off the reflective surface 60associated with the larger electrode 40 and the capsule 20 appears lightin color, e.g. white (see FIG. 2A). When the particles 50 are moved tothe larger electrode 40, the reflecting surface 60 is obscured and thecapsule 20 appears dark (see FIG. 2B) because light is absorbed by theparticles 50 before reaching the reflecting surface 60. The reflectingsurface 60 for the larger electrode 40 may possess retroflectiveproperties, specular reflection properties, diffuse reflectiveproperties or gain reflection properties. In certain embodiments, thereflective surface 60 reflects light with a Lambertian distribution. Thesurface 60 may be provided as a plurality of glass spheres disposed onthe electrode 40, a diffractive reflecting layer such as aholographically formed reflector, a surface patterned to totallyinternally reflect incident light, a brightness-enhancing film, adiffuse reflecting layer, an embossed plastic or metal film, or anyother known reflecting surface. The reflecting surface 60 may beprovided as a separate layer laminated onto the larger electrode 40 orthe reflecting surface 60 may be provided as a unitary part of thelarger electrode 40. In the embodiments depicted by FIGS. 2C and 2D, thereflecting surface may be disposed below the electrodes 30, 40 vis-à-visthe viewpoint 10. In these embodiments, electrode 30 should betransparent so that light may be reflected by surface 60. In otherembodiments, proper switching of the particles may be accomplished witha combination of alternating-current (AC) and direct-current (DC)electric fields and described below in connection with FIGS. 3A-3D.

In still other embodiments, the rear-addressed display previouslydiscussed can be configured to transition between largely transmissiveand largely opaque modes of operation (referred to hereafter as “shuttermode”). Referring back to FIGS. 1A and 1B, in these embodiments thecapsule 20 contains at least one positively-charged particle 50dispersed in a substantially clear dispersing fluid 25. The largerelectrode 40 is transparent and the smaller electrode 30 is opaque. Whenthe smaller, opaque electrode 30 is placed at a negative voltagepotential relative to the larger, transmissive electrode 40, theparticles 50 migrate to the smaller, opaque electrode 30. The effect toa viewer of the capsule 20 located at position 10 is a mixture of thelarger, transparent electrode 40 and the smaller, opaque electrode 30,creating an effect which is largely transparent. Referring to FIG. 1B,when the smaller, opaque electrode 30 is placed at a positive voltagepotential relative to the larger, transparent electrode 40, particles 50migrate to the second electrode 40 and the viewer is presented a mixtureof the opaque particles 50 covering the larger, transparent electrode 40and the smaller, opaque electrode 30, creating an effect which islargely opaque. In this manner, a display formed using the capsulesdepicted in FIGS. 1A and 1B may be switched between transmissive andopaque modes. Such a display can be used to construct a window that canbe rendered opaque. Although FIGS. 1A-2D depict a pair of electrodesassociated with each capsule 20, it should be understood that each pairof electrodes may be associated with more than one capsule 20.

A similar technique may be used in connection with the embodiment ofFIGS. 3A, 3B, 3C, and 3D. Referring to FIG. 3A, a capsule 20 contains atleast one dark or black particle 50 dispersed in a substantially cleardispersing fluid 25. A smaller, opaque electrode 30 and a larger,transparent electrode 40 apply both direct-current (DC) electric fieldsand alternating-current (AC) fields to the capsule 20. A DC field can beapplied to the capsule 20 to cause the particles 50 to migrate towardsthe smaller electrode 30. For example, if the particles 50 arepositively charged, the smaller electrode is placed a voltage that ismore negative than the larger electrode 40. Although FIGS. 3A-3D depictonly one capsule per electrode pair, multiple capsules may be addressedusing the same electrode pair.

The smaller electrode 30 is at most one-half the size of the largerelectrode 40. In preferred embodiments the smaller electrode isone-quarter the size of the larger electrode 40; in more preferredembodiments the smaller electrode 30 is one-eighth the size of thelarger electrode 40. In even more preferred embodiments, the smallerelectrode 30 is one-sixteenth the size of the larger electrode 40.

Causing the particles 50 to migrate to the smaller electrode 30, asdepicted in FIG. 3A, allows incident light to pass through the larger,transparent electrode 40 and be reflected by a reflecting surface 60. Inshutter mode, the reflecting surface 60 is replaced by a translucentlayer, a transparent layer, or a layer is not provided at all, andincident light is allowed to pass through the capsule 20, i.e. thecapsule 20 is transmissive.

Referring now to FIG. 3B, the particles 50 are dispersed into thecapsule 20 by applying an AC field to the capsule 20 via the electrodes30, 40. The particles 50, dispersed into the capsule 20 by the AC field,block incident light from passing through the capsule 20, causing it toappear dark at the viewpoint 10. The embodiment depicted in FIGS. 3A-3Bmay be used in shutter mode by not providing the reflecting surface 60and instead providing a translucent layer, a transparent layer, or nolayer at all. In shutter mode, application of an AC electric fieldcauses the capsule 20 to appear opaque. The transparency of a shuttermode display formed by the apparatus depicted in FIGS. 3A-3D may becontrolled by the number of capsules addressed using DC fields and ACfields. For example, a display in which every other capsule 20 isaddressed using an AC field would appear fifty percent transmissive.

FIGS. 3C and 3D depict an embodiment of the electrode structuredescribed above in which electrodes 30, 40 are on “top” of the capsule20, that is, the electrodes 30, 40 are between the viewpoint 10 and thecapsule 20. In these embodiments, both electrodes 30, 40 should betransparent. Transparent polymers can be fabricated using conductivepolymers, such as polyaniline, polythiophenes, or indium tin oxide.These materials may be made soluble so that electrodes can be fabricatedusing coating techniques such as spin coating, spray coating, meniscuscoating, printing techniques, forward and reverse roll coating and thelike. In these embodiments, light passes through the electrodes 30, 40and is either absorbed by the particles 50, reflected by retroreflectinglayer 60 (when provided), or transmitted throughout the capsule 20 (whenretroreflecting layer 60 is not provided).

The addressing structure depicted in FIGS. 3A-3D may be used withelectrophoretic display media and encapsulated electrophoretic displaymedia. FIGS. 3A-3D depict embodiments in which electrode 30, 40 arestatically attached to the display media. In certain embodiments, theparticles 50 exhibit bistability, that is, they are substantiallymotionless in the absence of a electric field. In these embodiments, theelectrodes 30, 40 may be provided as part of a “stylus” or other devicewhich is scanned over the material to address each capsule or cluster ofcapsules. This mode of addressing particle-based displays will bedescribed in more detail below in connection with FIG. 16.

Referring now to FIGS. 4A and 4B, a capsule 20 of a electronicallyaddressable media is illustrated in which the technique illustratedabove is used with multiple rear-addressing electrodes. The capsule 20contains at least one particle 50 dispersed in a clear suspending fluid25. The capsule 20 is addressed by multiple smaller electrodes 30 andmultiple larger electrodes 40. In these embodiments, the smallerelectrodes 30 should be selected to collectively be at most one-half thesize of the larger electrodes 40. In further embodiments, the smallerelectrodes 30 are collectively one-fourth the size of the largerelectrodes 40. In further embodiments the smaller electrodes 30 arecollectively one-eighth the size of the larger electrodes 40. Inpreferred embodiments, the smaller electrodes 30 are collectivelyone-sixteenth the size of the larger electrodes. Each electrode 30 maybe provided as separate electrodes that are controlled in parallel tocontrol the display. For example, each separate electrode may besubstantially simultaneously set to the same voltage as all otherelectrodes of that size. Alternatively, the electrodes 30, 40 may beinterdigitated to provide the embodiment shown in FIGS. 4A and 4B.

Operation of the rear-addressing electrode structure depicted in FIGS.4A and 4B is similar to that described above. For example, the capsule20 may contain positively charged, black particles 50 dispersed in asubstantially clear suspending fluid 25. The smaller electrodes 30 arecolored black and the larger electrodes 40 are colored white or arehighly reflective. Referring to FIG. 4A, the smaller electrodes 30 areplaced at a negative potential relative to the larger electrodes 40,causing particles 50 migrate within the capsule to the smallerelectrodes 30 and the capsule 20 appears to the viewpoint 10 as a mix ofthe larger, white electrodes 40 and the smaller, black electrodes 30,creating an effect which is largely white. Referring to FIG. 4B, whenthe smaller electrodes 30 are placed at a positive potential relative tothe larger electrodes 40, particles 50 migrate to the larger electrodes40 causing the capsule 20 to display a mix of the larger, whiteelectrodes 40 occluded by the black particles 50 and the smaller, blackelectrodes 30, creating an effect which is largely black. The techniquesdescribed above with respect to the embodiments depicted in FIGS. 1A and1B for producing two-color displays work with equal effectiveness inconnection with these embodiments.

FIGS. 5A and 5B depict an embodiment of a rear-addressing electrodestructure that creates a reflective color display in a manner similar tohalftoning or pointillism. The capsule 20 contains white particles 55dispersed in a clear suspending fluid 25. Electrodes 42, 44, 46, 48 arecolored cyan, magenta, yellow, and white respectively. Referring to FIG.5A, when the colored electrodes 42, 44, 46 are placed at a positivepotential relative to the white electrode 48, negatively-chargedparticles 55 migrate to these three electrodes, causing the capsule 20to present to the viewpoint 10 a mix of the white particles 55 and thewhite electrode 48, creating an effect which is largely white. Referringto FIG. 5B, when electrodes 42, 44, 46 are placed at a negativepotential relative to electrode 48, particles 55 migrate to the whiteelectrode 48, and the eye 10 sees a mix of the white particles 55, thecyan electrode 42, the magenta electrode 44, and the yellow electrode46, creating an effect which is largely black or gray. By addressing theelectrodes, any color can be produced that is possible with asubtractive color process. For example, to cause the capsule 20 todisplay an orange color to the viewpoint 10, the yellow electrode 46 andthe magenta electrode 42 are set to a voltage potential that is morepositive than the voltage potential applied by the cyan electrode 42 andthe white electrode 48. Further, the relative intensities of thesecolors can be controlled by the actual voltage potentials applied to theelectrodes.

In another embodiment, depicted in FIG. 6, a color display is providedby a capsule 20 of size d containing multiple species of particles in aclear, dispersing fluid 25. Each species of particles has differentoptical properties and possess different electrophoretic mobilities (μ)from the other species. In the embodiment depicted in FIG. 6, thecapsule 20 contains red particles 52, blue particles 54, and greenparticles 56, and|μ_(R)|

|μ_(B)|

|μ_(G)|That is, the magnitude of the electrophoretic mobility of the redparticles 52, on average, exceeds the electrophoretic mobility of theblue particles 54, on average, and the electrophoretic mobility of theblue particles 54, on average, exceeds the average electrophoreticmobility of the green particles 56. As an example, there may be aspecies of red particle with a zeta potential of 100 millivolts (mV), ablue particle with a zeta potential of 60 mV, and a green particle witha zeta potential of 20 mV. The capsule 20 is placed between twoelectrodes 32, 42 that apply an electric field to the capsule.

FIGS. 7A-7B depict the steps to be taken to address the display shown inFIG. 6 to display a red color to a viewpoint 10. Referring to FIG. 7A,all the particles 52, 54, 56 are attracted to one side of the capsule 20by applying an electric field in one direction. The electric fieldshould be applied to the capsule 20 long enough to attract even the moreslowly moving green particles 56 to the electrode 34. Referring to FIG.7B, the electric field is reversed just long enough to allow the redparticles 52 to migrate towards the electrode 32. The blue particles 54and green particles 56 will also move in the reversed electric field,but they will not move as fast as the red particles 52 and thus will beobscured by the red particles 52. The amount of time for which theapplied electric field must be reversed can be determined from therelative electrophoretic mobilities of the particles, the strength ofthe applied electric field, and the size of the capsule.

FIGS. 8A-8D depict addressing the display element to a blue state. Asshown in FIG. 8A, the particles 52, 54, 56 are initially randomlydispersed in the capsule 20. All the particles 52, 54, 56 are attractedto one side of the capsule 20 by applying an electric field in onedirection (shown in FIG. 8B). Referring to FIG. 8C, the electric fieldis reversed just long enough to allow the red particles 52 and blueparticles 54 to migrate towards the electrode 32. The amount of time forwhich the applied electric field must be reversed can be determined fromthe relative electrophoretic mobilities of the particles, the strengthof the applied electric field, and the size of the capsule. Referring toFIG. 8D, the electric field is then reversed a second time and the redparticles 52, moving faster than the blue particles 54, leave the blueparticles 54 exposed to the viewpoint 10. The amount of time for whichthe applied electric field must be reversed can be determined from therelative electrophoretic mobilities of the particles, the strength ofthe applied electric field, and the size of the capsule.

FIGS. 9A-9C depict the steps to be taken to present a green display tothe viewpoint 10. As shown in FIG. 9A, the particles 52, 54, 56 areinitially distributed randomly in the capsule 20. All the particles 52,54, 56 are attracted to the side of the capsule 20 proximal theviewpoint 10 by applying an electric field in one direction. Theelectric field should be applied to the capsule 20 long enough toattract even the more slowly moving green particles 56 to the electrode32. As shown in FIG. 9C, the electric field is reversed just long enoughto allow the red particles 52 and the blue particles 54 to migratetowards the electrode 54, leaving the slowly-moving green particles 56displayed to the viewpoint. The amount of time for which the appliedelectric field must be reversed can be determined from the relativeelectrophoretic mobilities of the particles, the strength of the appliedelectric field, and the size of the capsule.

In other embodiments, the capsule contains multiple species of particlesand a dyed dispersing fluid that acts as one of the colors. In stillother embodiments, more than three species of particles may be providedhaving additional colors. Although FIGS. 6-9C depict two electrodesassociated with a single capsule, the electrodes may address multiplecapsules or less than a full capsule.

FIGS. 10A-10H illustrate a capsule 120 having a capsule wall 124 andcontaining three different species of particles differing in color andelectrophoretic mobility and dispersed in a colored fluid 125. As inFIGS. 6-9, the capsule 120 is provided with light transmissive front andrear electrodes 32 and 34 respectively on opposed sides thereof, withthe front electrode 32 providing the viewing surface of the capsule.More specifically, the capsule 120 comprises negatively charged whiteparticles (denoted W−), and positively charged cyan and magentaparticles, with the cyan particles (denoted +C+) having a higherelectrophoretic mobility than the magenta particles (denoted M+). Thefluid 125 is colored with a yellow dye. The concentration of yellow dyeshould be chosen such that the yellow optical state of the display(described below with reference to FIG. 10D) provides a sufficientlysaturated yellow color, but the yellow does not substantiallycontaminate other colors when electrophoretic particles lie adjacent thefront electrode 32. The white W−, cyan +C+ and magenta M+ particles areall reflective. The yellow color of the dyed fluid is apparent only whenthere are no electrophoretic particles adjacent the front electrode 32.For example, if the white particles W− are driven adjacent the frontelectrode 32, the yellow color of the fluid 125 is not visible becausethe path of light (which enters through the front electrode 32, isreflected from the white particles W− and passes back through the frontelectrode 32) through the colored fluid is very short. If, however, thewhite particles W− are spaced from the front electrode 32 by asufficient display (perhaps ¼ of the thickness of the fluid layer) theyellow color of the dyed fluid 125 will become visible as the path ofreflected light through the fluid becomes substantial. The effect issimilar to that in prior art single particle/dyed fluid electrophoreticdisplays.

As already noted, the cyan +C+ and magenta M+ particles are bothpositively charged but have differing electrophoretic mobilities; thepresent description will assume that the cyan particles have the highermobility but obviously the reverse could be the case.

The capsule 120 is capable of displaying white, cyan, magenta, yellow,red, green, blue and black colors at its viewing surface (the frontelectrode 32), as illustrated in FIGS. 10A-10G respectively. To displaya white color, the rear electrode 34 is simply made negative relative tothe front electrode 32 for an extended period (all referenceshereinafter to making the rear electrode 34 negative or positive referto making this rear electrode negative or positive relative to the frontelectrode 32, since typically in practice the front electrode 32 will bea common front electrode extending across the whole display, while therear electrode 34 will be one of a multitude of individuallycontrollable pixel electrodes), so that the white particles W− lieadjacent the front electrode 32 and the cyan +C+ and magenta M+particles lie adjacent the rear electrode 34. In this situation, thewhite particles W− mask the cyan +C+ and magenta M+ particles and theyellow color of the fluid 125 (as previously noted, the pass length oflight through the fluid 125 is too short for any appreciablecontamination of the white color of the white particles W− by the yellowcolor of the fluid), so that a white color is displayed at the viewingsurface of the display.

To produce a cyan color, as illustrated in FIG. 10B, one first applies anegative pulse to rear electrode 34 (which brings about substantiallythe same situation as in FIG. 10A, with the white particles W− adjacentthe front electrode 32 and the cyan +C+ and magenta M+ particlesadjacent the rear electrode 34), followed by a positive pulse shorterthan the negative pulse. The positive pulse causes the white particlesW− to approach the rear electrode 34 and both the cyan +C+ and magentaM+ particles to approach the front electrode 32. However, because of thegreater mobility of the cyan +C+ particles, they approach the frontelectrode 32 more rapidly and the length of the positive pulse is chosenso that the cyan +C+ particles reach the front electrode 32 but themagenta particles M+ do not; in colloquial terms, the cyan particles“outrace” the magenta particles. In the situation shown in FIG. 10B, thecyan particles +C+ mask the magenta M+ and white W− particles and theyellow color of the fluid 125 (as previously noted, the pass length oflight through the fluid 125 is too short for any appreciablecontamination of the cyan color of the cyan particles +C+ by the yellowcolor of the fluid), so that a cyan color is displayed at the viewingsurface of the display.

To produce a magenta color, as illustrated in FIG. 10C, one firstapplies a long positive pulse, which brings both the cyan particles +C+and the magenta particles M+ adjacent the front electrode 32 and thewhite particles adjacent the rear electrode 34. There is then applied avery short negative pulse, which causes both the cyan particles +C+ andthe magenta particles M+ to move away from the front electrode 32.However, because of the greater mobility of the cyan particles +C+, theymove away from the front electrode 32 more rapidly than the magentaparticles M+, leaving the magenta particles visible through the frontelectrode 32 and screening the cyan particles C+, the white particles W−and the yellow color of the fluid 125. The duration of the shortnegative pulse is chosen such that the pass length of light through thefluid 125 is too short for any appreciable contamination of the magentacolor of the magenta particles M+ by the yellow color of the fluid. Theshort negative pulse also, of course, causes the white particles W− tomove away from the rear electrode 34 but this has no effect on the colordisplayed.

To produce a yellow color, as illustrated in FIG. 10D, one first appliesa negative pulse, which brings about substantially the same situation asin FIG. 10A, with the white particles W−adjacent the front electrode 32and the cyan +C+ and magenta M+ particles adjacent the rear electrode34. One then applies a positive pulse, shorter than the negative pulse,to cause the white particles W− to move away from the front electrode 32and the cyan +C+ and magenta particles to move away from the rearelectrode 34. The length of the positive pulse is controlled so that thewhite particles W− remain closer to the front electrode 32 than the cyan+C+ and magenta particles but such that there is a substantial distancebetween the white particles W− and the front electrode 32. Thus, asillustrated in FIG. 10D, the white particles W− mask the cyan particles+C+ and the magenta particles M+. However, unlike the situation in FIG.10A, in FIG. 10D the white particles are spaced a substantially distancefrom the front electrode 32 and act as a diffuse reflector causing lightentering through the front electrode 32 and passing through the yellowfluid 125 to be reflected back through the yellow fluid 125 and thefront electrode 32. Since this light has a substantial pass lengththrough the yellow fluid 125, a yellow color is displayed.

To display a red state, as illustrated in FIG. 10E, one first applies arelatively long positive pulse which, like the long positive pulse usedin FIG. 10C, brings the cyan +C+ and magenta M+ particles adjacent thefront electrode 32 and the white particles W− adjacent the rearelectrode 34. Next, a negative pulse shorter than the initial positivepulse but longer than the negative pulse applied in FIG. 10C, isapplied, and, for the same reasons as in FIG. 10C, causes the magentaparticles M+ to be closest to the front electrode 32 and to mask thecyan particles +C+ and the white particles W−. However, the finalnegative pulse still leaves the magenta particles M+ substantiallyspaced from the front electrode 32, so that, for reasons similar tothose discussed above in relation to FIG. 10D, the appearance of thedisplay is affected by the yellow dye through which light reflected fromthe magenta particles M+ passes, and thus the appearance of the displayis a combination of yellow dye absorption and magenta reflection, givinga red appearance.

To display a green state, as illustrated in FIG. 10F, one first appliesa relatively long negative pulse which, like the long negative pulseused in FIG. 10A, brings the white particles W− particles adjacent thefront electrode 32 and the cyan +C+ and magenta M+ adjacent the rearelectrode 34. Next, a very short positive pulse is applied. Thispositive pulse causes the cyan particles +C+ to move forwardly untilthey lie forward of the white particles W−, which of course movebackwardly from the front electrode 32. The positive pulse also causesthe magenta particles M+ to move forwardly, but at a slower rate thanthe cyan particles +C+. The final situation is similar to that shown inFIG. 10B, in as much as the cyan particles +C+ lie closest to the frontelectrode 32 and mask the white particles W− and the magenta particlesM+. However, in the situation shown in FIG. 10F, the cyan particles arespaced from the front electrode 32 by a distance sufficient to causesubstantial absorption by the yellow dye present in the fluid 125.Hence, for reasons similar to those already discussed with reference toFIG. 10E, the appearance of the display in FIG. 10F is a combination ofyellow dye absorption and cyan reflection, giving a green appearance.

To display a blue state, as illustrated in FIG. 10G, one applies a longpositive pulse which, like the long positive pulse used in FIG. 10C andthe first pulse used in FIG. 10E, brings the cyan +C+ and magenta M+particles adjacent the front electrode 32 and the white particlesW−adjacent the rear electrode 34. Note that in the situation shown inFIG. 10G two different reflection mechanisms are at work. If light isreflected only from a single particle, the mixtures of reflections fromcyan and magenta particles will appear to the eye as a light blue. If,however, light is reflected by at least one cyan particle and onemagenta particle, the light will appear a deeper blue. Since it can beshown that much of the light scattering from electrophoretic mediainvolves multiple reflections, the situation shown in FIG. 10G willprovide a well saturated blue.

Finally, to display a black state, as illustrated in FIG. 10H, oneapplies a long positive pulse, which produces the situation shown inFIG. 10G, and then applies a short negative pulse. The short negativepulse moves the cyan +C+ and magenta M+ particles away from the frontelectrode 32 thus (for reasons similar to those already discussed withreference to FIGS. 10D, 10E and 10F) admixing the yellow color of thefluid 125 with the blue reflection shown in FIG. 10G and producing aprocess black appearance.

FIGS. 11A-11H illustrate a display generally similar to that illustratedin FIGS. 10A-10H but in which the cyan particles +C+ and the magentaparticles M+ are transmissive rather than reflective. The use oftransmissive rather than reflective particles requires somemodifications of the necessary positions of the particles in certainoptical states because a transmissive colored particle does not screenout the colors of particles “further back” (i.e., closer to the rearelectrode 34) and hence in some optical states it is necessary tocontrol carefully the positions of the white particles W− in order toensure that such screening does occur.

FIG. 11A shows the white state of the display. This white state isidentical to that shown in FIG. 10A and is reached in exactly the samemanner; since the white particles W− hide both the cyan particles +C+and the magenta particles M+ in this state of the display, the use oftransmissive cyan and magenta particles rather than reflective particlesmakes no difference to the appearance of this state of the display.

FIG. 11B shows the cyan state of the display. This state of the displaydiffers from that shown in FIG. 10B in that the white particles W− needto be disposed immediately behind the cyan particles +C+ in order thatthe white particles can screen the magenta particles M+. Light enteringthe display through the front electrode 32 passes through thetransmissive cyan particles, is reflected from the white particles, andthen passes back through the cyan particles and back out of the displaythrough the front electrode. To avoid contaminating the cyan color thusproduced with yellow (and thus shifting the displayed color towardsgreen), it is important that the white particles be close behind thecyan particles, so that the light travelling the aforesaid path does nothave to travel a significant distance through the yellow fluid 125.

Provided that the electrophoretic mobility of the cyan particles +C+ ismuch greater than that of the magenta particles M+, and the absolutevalues of the electrophoretic mobilities of the magenta and whiteparticles are comparable, the display state shown in FIG. 11B can beproduced by first driving the display to the state shown in FIG. 11A andthen applying to the rear electrode 34 a positive pulse just sufficientto drive the cyan particles to the front electrode 32 and the whiteparticles a short distance away from this front electrode.

FIG. 11C shows the magenta optical state of the display. This isgenerally similar to the cyan optical state shown in FIG. 11B, but withthe magenta particles adjacent the front electrode 32 and the cyanparticles adjacent the rear electrode 34. The magenta optical statefunctions in a manner exactly parallel to the cyan optical state; lightentering the display through the front electrode 32 passes through thetransmissive magenta particles, is reflected from the white particles,and then passes back through the magenta particles and back out of thedisplay through the front electrode. Again, to avoid contaminating themagenta color thus produced with yellow (and thus shifting the displayedcolor towards red), it is important that the white particles be closebehind the magenta particles, so that the light travelling the aforesaidpath does not have to travel a significant distance through the yellowfluid 125.

FIG. 11D shows the yellow optical state of the display. This isidentical to the yellow state shown in FIG. 10D, can be produced usingthe same drive pulses, and the yellow color is produced in the samemanner; light entering the display through the front electrode 32 passesthrough the yellow fluid 125, is reflected from the white particles,passes back through the yellow fluid 125 and back through the frontelectrode 32.

FIG. 11E shows the red optical state of the display. The positions ofthe particles in this red optical state are identical to those of thesimilar red state shown in FIG. 10E, and the red state can be broughtabout using the same drive pulses as in FIG. 10E. However, the actualmanner in which the red color is produced in FIG. 11E differs slightlyfrom that described with reference to FIG. 10E. In FIG. 11E, lightentering the display through the front electrode 32 passes through theyellow fluid 125 and the transmissive magenta particles, is reflectedfrom the white particles, passes back through the magenta particles andthe yellow fluid 125 and back through the front electrode 32 to producea red appearance to the display.

FIG. 11F shows the green optical state of the display. The positions ofthe particles in this green optical state are identical to those of thesimilar green state shown in FIG. 10F, and the green state can bebrought about using the same drive pulses as in FIG. 10F. However, aswith the red optical state shown in FIG. 11E, the actual manner in whichthe green color is produced in FIG. 11F differs slightly from thatdescribed with reference to FIG. 10F. In FIG. 11F, light entering thedisplay through the front electrode 32 passes through the yellow fluid125 and the transmissive cyan particles, is reflected from the whiteparticles, passes back through the cyan particles and the yellow fluid125 and back through the front electrode 32 to produce a greenappearance to the display.

FIG. 11G shows the blue optical state of the display, which differs fromthe corresponding blue state shown in FIG. 10G in that the whiteparticles are located relatively close to the front electrode,immediately behind the mixed layer of cyan and magenta particles. InFIG. 11G, light entering the display through the front electrode 32passes through the transmissive magenta and cyan particles, is reflectedfrom the white particles, passes back through the magenta and cyanparticles and back through the front electrode 32 to produce a blueappearance to the display.

Finally, FIG. 11H shows one possible black state of the display, thisblack state being identical, as to particle position to that shown inFIG. 10H. However, the way in which the black state is produced isslightly different from that described above with regard to FIG. 10H. InFIG. 11H, light entering the display through the front electrode 32passes through the transmissive magenta and cyan particles and theyellow fluid 125, so that essentially all light is absorbed before itcan reach the white particles adjacent the rear electrode 34. Any lightwhich does reach the white particles will be reflected back and againpass through the transmissive magenta and cyan particles and the yellowfluid 125, so that essentially no light will re-emerge from the frontelectrode 32, and a black optical state will be displayed. It should benoted that in this black optical state, there is considerable freedom asregards the disposition of the magenta and cyan particles, provided bothtypes of particles lie closer to the front electrode than the whiteparticles; since the yellow fluid 125 and the magenta and cyan particlesare all transmissive, the exact order in which incoming light encountersthe fluid and the two types of particles is essentially irrelevant, andhence the positions of the magenta and cyan particles can be variedprovided both lie closer to the front electrode than the whiteparticles. For example, in the display shown in FIGS. 11A-11H, theparticle positions shown in FIG. 10G would provide a black opticalstate.

It will be seen from the foregoing that the displays illustrated inFIGS. 10A-10H and 11A-11H are capable of displaying white, black, cyan,magenta, yellow, red, green and blue colors over their entire displayareas. As previously noted, displays using RGB color filter arrays arecapable of displaying red, green and blue colors over only one third oftheir display area, black over the whole display area and a processwhite equivalent to white over one third of the display area. Similarly,displays using RGBW color filter arrays are capable of displaying red,green and blue colors over only one fourth of their display area, blackover the whole display area and a process white equivalent to white overone half of the display area. The white states of the displaysillustrated in FIGS. 10A-10H and 11A-11H should thus be dramaticallybetter than that of any display based upon color filters, and the red,green and blue states should also be improved. Furthermore, the whitestates of the displays illustrated in FIGS. 10A-10H and 11A-11H shouldbe dramatically better than that of the multi-particle displayillustrated in FIGS. 6-9, which relies upon a process white stateequivalent to white over one third of the display area.

In certain cases, it may be difficult to procure colored particleshaving the desired colors and relative electrophoretic mobilities neededto enable each of the optical states shown in FIGS. 10A-10H or 11A-11Hto be achieved using simple sets of drive pulses. In such cases, it maybe appropriate to use at least one type of particle which has anelectrophoretic mobility which varies with applied voltage, so that therelative electrophoretic mobilities of two types of particles can bevaried by adjusting the driving voltage used, as described in U.S.Patent Publication No. 2006/0202949, assigned to the same assignee asthe present application. Since the particles used in the displays of thepresent invention may have voltage-dependent mobilities, referencesherein the particles having differing electrophoretic mobilities shouldbe understood as including particles having differing electrophoreticmobilities at at least one driving voltage used in the displaycontaining the particles.

In FIG. 12, the rear substrate 100 for a seven segment display is shownthat improves on normal rear electrode structures by providing a meansfor arbitrarily connecting to any electrode section on the rear of thedisplay without the need for conductive trace lines on the surface ofthe patterned substrate or a patterned counter electrode on the front ofthe display. Small conductive vias through the substrate allowconnections to the rear electrode structure. On the back of thesubstrate, these vias are connected to a network of conductors. Theseconductors can be run so as to provide a simple connection to the entiredisplay. For example, segment 112 is connected by via 114 through thesubstrate 116 to conductor 118. A network of conductors may run multipleconnections (not shown) to edge connector 122. This connector can bebuilt into the structure of the conductor such as edge connector 122.Each segment of the rear electrode can be individually addressed easilythrough edge connector 122. A continuous top electrode can be used withthe substrate 116.

The rear electrode structure depicted in FIG. 12 is useful for anydisplay media, but is particularly advantageous for particle-baseddisplays because such displays do not have a defined appearance when notaddressed. The rear electrode should be completely covered in anelectrically conducting material with room only to provide necessaryinsulation of the various electrodes. This is so that the connections onthe rear of the display can be routed with out concern for affecting theappearance of the display. Having a mostly continuous rear electrodepattern assures that the display material is shielded from the rearelectrode wire routing.

In FIG. 13, a 3×3 matrix is shown. Here, matrix segment 124 on a firstside of substrate 116 is connected by via 114 to conductor 118 on asecond side of substrate 116. The conductors 18 run to an edge andterminate in a edge connector 122. Although the display element of FIG.13 shows square segments 124, the segments may be shaped or sized toform a predefined display pattern.

In FIG. 14, a printed circuit board 138 is used as the rear electrodestructure. The front of the printed circuit board 138 has copper pads132 etched in the desired shape. There are plated vias 114 connectingthese electrode pads to an etched wire structure 136 on the rear of theprinted circuit board 138. The wires 136 can be run to one side or therear of the printed circuit board 138 and a connection can be made usinga standard connector such as a surface mount connector or using a flexconnector and anisotropic glue (not shown). Vias may be filled with aconductive substance, such as solder or conductive epoxy, or aninsulating substance, such as epoxy.

Alternatively, a flex circuit such a copper-clad polyimide may be usedfor the rear electrode structure of FIG. 14. Printed circuit board 138may be made of polyimide, which acts both as the flex connector and asthe substrate for the electrode structure. Rather than copper pads 132,electrodes (not shown) may be etched into the copper covering thepolyimide printed circuit board 138. The plated through vias 114 connectthe electrodes etched onto the substrate the rear of the printed circuitboard 138, which may have an etched conductor network thereon (theetched conductor network is similar to the etched wire structure 136).

In FIG. 15, a thin dielectric sheet 150, such as polyester, polyimide,or glass can be used to make a rear electrode structure. Holes 152 arepunched, drilled, abraded, or melted through the sheet where conductivepaths are desired. The front electrode 154 is made of conductive inkprinted using any technique described above. The holes should be sizedand the ink should be selected to have a viscosity so that the ink fillsthe holes. When the back structure 156 is printed, again usingconductive ink, the holes are again filled. By this method, theconnection between the front and back of the substrate is madeautomatically.

In FIG. 16, the rear electrode structure can be made entirely of printedlayers. A conductive layer 166 can be printed onto the back of a displaycomprised of a clear, front electrode 168 and a printable displaymaterial 170. A clear electrode may be fabricated from indium tin oxideor conductive polymers such as polyanilines and polythiophenes. Adielectric coating 176 can be printed leaving areas for vias. Then, theback layer of conductive ink 178 can be printed. If necessary, anadditional layer of conductive ink can be used before the final inkstructure is printed to fill in the holes.

This technique for printing displays can be used to build the rearelectrode structure on a display or to construct two separate layersthat are laminated together to form the display. For example anelectronically active ink may be printed on an indium tin oxideelectrode. Separately, a rear electrode structure as described above canbe printed on a suitable substrate, such as plastic, polymer films, orglass. The electrode structure and the display element can be laminatedto form a display.

Referring now to FIG. 17, a threshold may be introduced into anelectrophoretic display cell by the introduction of a third electrode.One side of the cell is a continuous, transparent electrode 200 (anode).On the other side of the cell, the transparent electrode is patternedinto a set of isolated column electrode strips 210. An insulator 212covers the column electrodes 210, and an electrode layer on top of theinsulator is divided into a set of isolated row electrode strips 230,which are oriented orthogonal to the column electrodes 210. The rowelectrodes 230 are patterned into a dense array of holes, or a grid,beneath which the exposed insulator 212 has been removed, forming amultiplicity of physical and potential wells.

A positively charged particle 50 is loaded into the potential wells byapplying a positive potential (e.g. 30V) to all the column electrodes210 while keeping the row electrodes 230 at a less positive potential(e.g. 15V) and the anode 200 at zero volts. The particle 50 may be aconformable capsule that situates itself into the physical wells of thecontrol grid. The control grid itself may have a rectangularcross-section, or the grid structure may be triangular in profile. Itcan also be a different shape which encourages the microcapsules tosituate in the grid, for example, hemispherical.

The anode 200 is then reset to a positive potential (e.g. 50V). Theparticle will remain in the potential wells due to the potentialdifference in the potential wells: this is called the Hold condition. Toaddress a display element the potential on the column electrodeassociated with that element is reduced, e.g. by a factor of two, andthe potential on the row electrode associated with that element is madeequal to or greater than the potential on the column electrode. Theparticles in this element will then be transported by the electric fielddue to the positive voltage on the anode 200. The potential differencebetween row and column electrodes for the remaining display elements isnow less than half of that in the normal Hold condition. The geometry ofthe potential well structure and voltage levels are chosen such thatthis also constitutes a Hold condition, i.e., no particles will leavethese other display elements and hence there will be no half-selectproblems. This addressing method can select and write any desiredelement in a matrix without affecting the pigment in any other displayelement. A control electrode device can be operated such that the anodeelectrode side of the cell is viewed.

The control grid may be manufactured through any of the processes knownin the art, or by several novel processes described herein. That is,according to traditional practices, the control grid may be constructedwith one or more steps of photolithography and subsequent etching, orthe control grid may be fabricated with a mask and a “sandblasting”technique.

In another embodiment, the control grid is fabricated by an embossingtechnique on a plastic substrate. The grid electrodes may be depositedby vacuum deposition or sputtering, either before or after the embossingstep. In another embodiment, the electrodes are printed onto the gridstructure after it is formed, the electrodes consisting of some kind ofprintable conductive material which need not be clear (e.g. a metal orcarbon-doped polymer, an intrinsically conducting polymer, etc.).

In a preferred embodiment, the control grid is fabricated with a seriesof printing steps. The grid structure is built up in a series of one ormore printed layers after the cathode has been deposited, and the gridelectrode is printed onto the grid structure. There may be additionalinsulator on top of the grid electrode, and there may be multiple gridelectrodes separated by insulator in the grid structure. The gridelectrode may not occupy the entire width of the grid structure, and mayonly occupy a central region of the structure, in order to stay withinreproducible tolerances. In another embodiment, the control grid isfabricated by photoetching away a glass, such as a photostructuralglass.

In an encapsulated electrophoretic image display, an electrophoreticsuspension, such as the ones described previously, is placed insidediscrete compartments that are dispersed in a polymer matrix. Thisresulting material is highly susceptible to an electric field across thethickness of the film. Such a field is normally applied using electrodesattached to either side of the material. However, as described above inconnection with FIGS. 3A-3D, some display media may be addressed bywriting electrostatic charge onto one side of the display material. Theother side normally has a clear or opaque electrode. For example, asheet of encapsulated electrophoretic display media can be addressedwith a head providing DC voltages.

In another implementation, the encapsulated electrophoretic suspensioncan be printed onto an area of a conductive material such as a printedsilver or graphite ink, aluminized Mylar, or any other conductivesurface. This surface which constitutes one electrode of the display canbe set at ground or high voltage. An electrostatic head consisting ofmany electrodes can be passed over the capsules to addressing them.Alternatively, a stylus can be used to address the encapsulatedelectrophoretic suspension.

In another implementation, an electrostatic write head is passed overthe surface of the material. This allows very high resolutionaddressing. Since encapsulated electrophoretic material can be placed onplastic, it is flexible. This allows the material to be passed throughnormal paper handling equipment. Such a system works much like aphotocopier, but with no consumables. The sheet of display materialpasses through the machine and an electrostatic or electrophotographichead addresses the sheet of material.

In another implementation, electrical charge is built up on the surfaceof the encapsulated display material or on a dielectric sheet throughfrictional or triboelectric charging. The charge can built up using anelectrode that is later removed. In another implementation, charge isbuilt up on the surface of the encapsulated display by using a sheet ofpiezoelectric material.

FIG. 18 shows an electrostatically written display. Stylus 300 isconnected to a positive or negative voltage. The head of the stylus 300can be insulated to protect the user. Dielectric layer 302 can be, forexample, a dielectric coating or a film of polymer. In otherembodiments, dielectric layer 302 is not provided and the stylus 300contacts the encapsulated electrophoretic display 304 directly.Substrate 306 is coated with a clear conductive coating such as ITOcoated polyester. The conductive coating is connected to ground. Thedisplay 304 may be viewed from either side.

Microencapsulated displays offer a useful means of creating electronicdisplays, many of which can be coated or printed. There are manyversions of microencapsulated displays, including microencapsulatedelectrophoretic displays. These displays can be made to be highlyreflective, bistable, and low power.

To obtain high resolution displays, it is useful to use some externaladdressing means with the microencapsulated material. This inventiondescribes useful combinations of addressing means with microencapsulatedelectrophoretic materials in order to obtain high resolution displays.

One method of addressing liquid crystal displays is the use ofsilicon-based thin film transistors to form an addressing backplane forthe liquid crystal. For liquid crystal displays, these thin filmtransistors are typically deposited on glass, and are typically madefrom amorphous silicon or polysilicon. Other electronic circuits (suchas drive electronics or logic) are sometimes integrated into theperiphery of the display. An emerging field is the deposition ofamorphous or polysilicon devices onto flexible substrates such as metalfoils or plastic films.

The addressing electronic backplane could incorporate diodes as thenonlinear element, rather than transistors. Diode-based active matrixarrays have been demonstrated as being compatible with liquid crystaldisplays to form high resolution devices.

There are also examples of crystalline silicon transistors being used onglass substrates. Crystalline silicon possesses very high mobilities,and thus can be used to make high performance devices. Presently, themost straightforward way of constructing crystalline silicon devices ison a silicon wafer. For use in many types of liquid crystal displays,the crystalline silicon circuit is constructed on a silicon wafer, andthen transferred to a glass substrate by a “liftoff” process.Alternatively, the silicon transistors can be formed on a silicon wafer,removed via a liftoff process, and then deposited on a flexiblesubstrate such as plastic, metal foil, or paper. As anotherimplementation, the silicon could be formed on a different substratethat is able to tolerate high temperatures (such as glass or metalfoils), lifted off, and transferred to a flexible substrate. As yetanother implementation, the silicon transistors are formed on a siliconwafer, which is then used in whole or in part as one of the substratesfor the display.

The use of silicon-based circuits with liquid crystals is the basis of alarge industry. Nevertheless, these display possess serious drawbacks.Liquid crystal displays are inefficient with light, so that most liquidcrystal displays require some sort of backlighting. Reflective liquidcrystal displays can be constructed, but are typically very dim, due tothe presence of polarizers. Most liquid crystal devices require precisespacing of the cell gap, so that they are not very compatible withflexible substrates. Most liquid crystal displays require a “rubbing”process to align the liquid crystals, which is both difficult to controland has the potential for damaging the TFT array.

The combination of these thin film transistors with microencapsulatedelectrophoretic displays should be even more advantageous than withliquid crystal displays. Thin film transistor arrays similar to thoseused with liquid crystals could also be used with the microencapsulateddisplay medium. As noted above, liquid crystal arrays typically requiresa “rubbing” process to align the liquid crystals, which can cause eithermechanical or static electrical damage to the transistor array. No suchrubbing is needed for microencapsulated displays, improving yields andsimplifying the construction process.

Microencapsulated electrophoretic displays can be highly reflective.This provides an advantage in high-resolution displays, as a backlightis not required for good visibility. Also, a high-resolution display canbe built on opaque substrates, which opens up a range of new materialsfor the deposition of thin film transistor arrays.

Moreover, the encapsulated electrophoretic display is highly compatiblewith flexible substrates. This enables high-resolution TFT displays inwhich the transistors are deposited on flexible substrates like flexibleglass, plastics, or metal foils. The flexible substrate used with anytype of thin film transistor or other nonlinear element need not be asingle sheet of glass, plastic, metal foil, though. Instead, it could beconstructed of paper. Alternatively, it could be constructed of a wovenmaterial. Alternatively, it could be a composite or layered combinationof these materials.

As in liquid crystal displays, external logic or drive circuitry can bebuilt on the same substrate as the thin film transistor switches.

In another implementation, the addressing electronic backplane couldincorporate diodes as the nonlinear element, rather than transistors.

In another implementation, it is possible to form transistors on asilicon wafer, dice the transistors, and place them in a large areaarray to form a large, TFT-addressed display medium. One example of thisconcept is to form mechanical impressions in the receiving substrate,and then cover the substrate with a slurry or other form of thetransistors. With agitation, the transistors will fall into theimpressions, where they can be bonded and incorporated into the devicecircuitry. The receiving substrate could be glass, plastic, or othernonconductive material. In this way, the economy of creating transistorsusing standard processing methods can be used to create large-areadisplays without the need for large area silicon processing equipment.

While the examples described here are listed using encapsulatedelectrophoretic displays, there are other particle-based display mediawhich should also work as well, including encapsulated suspendedparticles and rotating ball displays.

While the invention has been particularly shown and described withreference to specific preferred embodiments, it should be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

The invention claimed is:
 1. A method for displaying a plurality ofcolors in an electrophoretic display including an electrophoretic mediumdisposed between two electrodes, at least one of the electrodes beinglight-transmissive, the electrophoretic medium comprising a firstspecies of white reflective particles, a second species of transmissiveparticles, and a third species of transmissive particles, wherein thefirst, second, and third particles are different in color and havedifferent electrophoretic mobilities, the method comprising: driving thesecond species of transmissive particles adjacent the light-transmissiveelectrode and driving the first species of white reflective particlesbetween the second and third species of transmissive particles toachieve a first color; driving the third species of transmissiveparticles adjacent the light-transmissive electrode and driving thefirst species of white reflective particles between the second and thirdspecies of transmissive particles to achieve a second color; and drivingthe first, second, and third particles away from the light-transmissiveelectrode to achieve a third color.
 2. The method of claim 1, whereinthe second and third species of particles are cyan and magenta,respectively, in color.
 3. The method of claim 1, wherein the firstspecies of reflective particles are white in color.
 4. The method ofclaim 1, further comprising driving the first species of reflectiveparticles adjacent the light-transmissive electrode to achieve a fourthcolor.
 5. An electronic book reader, portable computer, tablet computer,cellular telephone, smart card, sign, watch, shelf label or flash driveincorporating the method of claim 1.